You need to be dosing,
To recommended on the bottle.
Do u get much algae with your lighting duration? If so drop it to 4 hours per period.
From the info given I'd say your plants are running out of nutrients and dying slowly. Excel is only liquid carbon. It increases plant uptake of nutrients but isn't a nutrient itself (well kind of is but isn't). So basically your giving your plants a boost to uptake nutrients that arnt available causing them to die of starvation.
Hope I've helped